This invention relates to the field of electrical connectors. More particularly, the present invention relates to electrical connectors for grounding large surge currents conducted by the braided shield of a multiple conductor cable.
Electronic controls for power distribution switchgear are considerably less resistant to damage and malfunction from overvoltage and overcurrent conditions than the switchgear they control. The electronic controls typically operate with voltages in the decades and currents in the same range or less. Distribution switchgear typically switches voltage in the decade kilovolt range and currents of hundreds of amperes or more. Distribution switchgear is often exposed to hazardous overvoltages and overcurrents due to a number of causes. A leading cause of such electrical hazards is lightning striking near the switchgear. To reduce the adverse effect of lightning, arrestors are frequently connected between the distribution line and ground to limit the overvoltage and shunt the surging overcurrent away from the protected equipment to ground. In accordance with good practice, the metal enclosed housings of the switchgear and the control are also grounded. An ideal grounding conductor would prevent the point grounded from ever rising above the ground voltage level, regardless of the current it conducts. They don't exist. Grounding conductors have finite current carrying capability and may melt, or overheat adjacent components unless considerable care is taken. Likewise, their finite conductivity allows a point connected to ground to rise in voltage, which can overstress the components to be protected.
The preferred method of grounding switchgear protected by an arrestor, and a control is to separately ground the arrestor, separately ground the switchgear and, ground the control through the grounding connection of the switchgear. Although the preferred grounding method is usually employed by users, national testing authorities require testing under a worst case approach to test resistance to a fault conditions. Arrestor flashover caused by lightning with associated surge currents are simulated by spark gap flashover. Worst case grounding (as set forth ANSI/IEEE specification C37.60-1981) is employed by connecting the arrestor to the grounding conductor of the switchgear. The control is separately grounded.
The grounding schemes thus far discussed relate to power grounds which are designed to carry significant levels of current when a fault occurs or an arrestor breaks over. It can reasonably be anticipated that the point to be grounded will, under fault conditions, rise several volts above ground. Such a faulted voltage rise would reck havoc in a control receiving and issuing signals in a similar voltage range. To avoid the confusion between valid signals and fault noise, a signal ground conductor between the control and associated switchgear is typically run separately from the power ground conductor to establsh a ground reference for signals. For older controls, one of the conductors in the cable connecting the switchgear and the control provided a signal ground.
More recently, as part of the trend to integrated circuits from discrete components, the voltage operating level of most of the control has been markedly decreased and signal processing speed increased. As a result, recent controls are more noise sensitive. Because a grounded conductor in a cable couples significant noise into adjacent conductors under fault conditions, using one of the conductors in a cable is not viable for the recent generation of controls.
A shielded cable with the shield grounded has been used to connect the switchgear and a recent type control. In addition to reducing the level of noise coupled into the conductors embraced by the braided shield under fault conditions, a grounded braided shield protects the conductors from adverse electric field conditions.
In one development, a prototype grounding shielded cable connector assembly included a cabinet connector, an aluminum plug, and a cable coupling. The cable shield was secured to a portion of the conductive cable coupling by a flat annular bronze spring and was also electrically secured to the cabinet connector by a jumpered connection. The principal path for ground current was through the cable coupling, into and out of a bronze current exchange spring bridging the annular space between the cable coupling and plug, and lastly through the plug to the threaded connection between the plug and a grounded cabinet connector. Unfortunately, the prototype assembly would not pass the ANSI/IEEE specification C37.60-1981, even when additional surge protection was incorporated into the cabinet connector. It is believed the failure of the prototype connector was caused by its use of three mechanical junctions, employing point contacts and dissimilar metals across junctions. Additionally the prototype was labor intensive.